A representative of prior art is JP2000-228807A, disclosing a combined discharging and grounding device is disclosed, which has a discharging unit with a first electric contact and a first counter-contact as well as a grounding unit with a second electric contact and a second counter contact. The first electric contact and the second electric contact are pro-vided on a pivotable arm member that is movable along a path of movement from an open position to a close position. The first electric contact is longer in dimension in the direction of the path of movement than the second contact such that an electric contact between the first electric contact and the first counter-contact is established in an intermediate position between the open position and the closed position of the movable member when closing the discharging and grounding device in an operating state of the discharging and grounding device while the second electric contact is electrically still disconnected from the second counter contact.
From similar devices it is known to connect the first electric contact via a discharge resistor to ground potential while the second electric contact is connected directly to ground potential. This way, the electric energy stored in a capacitor can be consumed by the discharge resistor prior to grounding it directly to ground potential when closing the combined discharging and grounding device. Direct grounding of the discharge resistors is a safety measure required for protecting an operator at the time of carrying out any maintenance operations in the electric device comprising the combined discharging and grounding device.
It is known that the full discharging of a capacitor or a capacitor bank in a classic high voltage device such as power converters requires quite some time, for example 30 to 40 minutes. Such a long time for grounding is dissatisfactory as it contributes substantially to long down times of the high voltage device. For that reason, there is an ever-rising demand for a quicker grounding procedure for fully grounding a complex device such as a multi-level power converter comprising several capacitors. Unfortunately, this contradicts the minimal required grounding time defined by the capacitors, in particular large capacitors in the millifarad range for high nominal voltages. The higher the rated current of a capacitors, the larger the overall cross-section of a conductor for discharging the capacitor and the longer the discharging time.